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TRANSCRIPT
ORIGINAL
Carbonization of reaction wood from Paulowniatomentosa and Pinus densiflora branch woods
Yue Qi1 • Jae-Hyuk Jang1,2 • Wahyu Hidayat1,3 •
Ae-Hee Lee1 • Seung-Hwan Lee1 • Hee-Mun Chae1 •
Nam-Hun Kim1
Received: 13 October 2015 / Published online: 7 May 2016
� Springer-Verlag Berlin Heidelberg 2016
Abstract The carbonization characteristics of tension wood (TW) and compression
wood (CW) from Paulownia tomentosa and Pinus densiflora branch woods,
respectively, were investigated and compared to those of opposite wood (OW) and
lateral wood (LW). The carbonization was conducted at 400, 600, and 800 �C.Heating values, pH, char yields, cell wall structures, and combustion characteristics
were different in TW and CW, whereas only little differences were found among
reaction wood, OW, and LW. The heating values of TW charcoal were lower than
those of CW charcoal at 400 and 600 �C, while the pH of TW charcoal was higher
than that of CW at all carbonization temperatures. The char yields of CW were the
highest among all the samples at all carbonization temperatures. Furthermore, the
TW charcoal was more thermally stable than CW charcoal. SEM observations
revealed significant differences in the cell wall structures of the woods before and
after carbonization. The gelatinous layer in TW disappeared, and the helical cavities
and intercellular spaces in CW persisted throughout the carbonization process. The
cell wall structures of the wood samples became smooth and amorphous as the
carbonization temperature increased.
& Nam-Hun Kim
1 College of Forest and Environmental Sciences, Kangwon National University,
Chuncheon 24341, Republic of Korea
2 The Institute of Forest Science, Kangwon National University, Chuncheon 24341, Republic of
Korea
3 Department of Forestry, Faculty of Agriculture, Lampung University, Bandar Lampung,
Indonesia
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Wood Sci Technol (2016) 50:973–987
DOI 10.1007/s00226-016-0828-y
Introduction
Charcoal as fuel has been conventionally applied to many fields, such as soil
modification, water and air purification, and smelting, and has recently refocused as
solid bio-energy. Wood, woody materials, woody wastes, agricultural wastes,
herbaceous plants, and biosolids are useful raw materials for charcoal production.
Many studies have been conducted to optimize carbonization conditions such as
temperature for wood charcoal production and have investigated carbonization
characteristics. Slocum et al. (1973) and Cutter and McGinnes (1981) have
extensively investigated the density change patterns in charred wood. According to
Slocum et al. (1973), the densities of oak and hickory charcoals were significantly
lower than those of the raw woods and were maximized at 600 �C. Cutter and
McGinnes (1981) reported that the density of seven wood species charcoal
decreased as the temperature increased to 600 �C, and the maximum rate of density
change was observed between 300 and 350 �C. Anatomical changes of the vessel
diameter, cell wall thickness, crystalline structure, and morphology in woods
subjected to carbonization were also reported by previous studies. McGinnes et al.
(1971), Kim and Hanna (2006) and Kumar and Gupta (1995) studied the
morphological changes of wood charcoal using scanning electronic microscope
(SEM). Elyounssi and Halim (2014) and Kwon et al. (2009) reported the
dimensional shrinkage and crystalline structure by an X-ray diffraction analysis.
Cutter et al. (1980) and Kwon et al. (2014) demonstrated the difference in cell wall
thickness before and after carbonization.
The chemical properties of charcoal have been studied in terms of ash chemical
composition, element content, and ultimate and proximate analyses (Kumar et al.
2009; Liu et al. 2014; Somerville and Jahanshahi 2015). Yang et al. (2007) reported
the degradation of chemical functional groups and a comprehensive understanding
of the pyrolysis of cellulose, hemicelluloses, and lignin with focus on the gas
product releasing properties. The pH change in wood charcoal as a function of the
carbonization temperature was also evaluated by Jo et al. (2009) and Kai et al.
(2000). They found that the pH was changed from acidic to basic as the
carbonization temperature increased.
Wood industry has mainly used stem wood, and most of the branches, especially
branch reaction wood, have been wasted. Reaction wood is undesirable in any
structural application, because it can twist, cup, or warp dramatically during
machining. Previous studies showed that the wood material containing reaction
wood had a reduced compressive and bending strength and a very low modulus of
elasticity in bending (Furuno et al. 1969). Therefore, efficient ways of utilizing the
branch wood need exploring.
Reaction wood can be classified as TW in hardwoods and CW in softwoods.
Reaction wood has specific characteristics, such as a gelatinous layer (G-layer) in
TW (Kaku et al. 2009; Lautner et al. 2012; Qi et al. 2014) and high lignin content in
CW (Timell 1986; Gindl 2002; Tarmian and Azadfallah 2009). Although a large
number of studies on wood carbonization have been performed, no study on the
carbonization characteristics of reaction wood has yet been reported to the authors’
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123
knowledge and searching. Furthermore, there is no available information, regarding
the carbonization characteristics of the cellulose-rich TW and lignin-rich CW.
Paulownia tomentosa is a fast-growing wood species with intensive branches,
and Pinus densiflora is one of the most common wood species in Korea. In a
previous study (Qi et al. 2014), the difference of the anatomical and physical
properties between TW, OW, and LW of Paulownia coreana was investigated to
contribute to efficient utilization of branch woods. This study investigated the
carbonization characteristics of branch wood, including reaction wood, to facilitate
its utilization as a carbonized material. The heating values, pH, char yields,
pyrolysis characteristics, and morphological changes of reaction wood charcoal
from P. tomentosa and P. densiflora were examined.
Materials and methods
Materials
Branch woods of P. tomentosa and P. densiflora were obtained from the Research
Forest of Kangwon National University (N 37�510/E 127�480) in South Korea. Air-
dried wood samples from P. tomentosawere categorized into TW, OW, and LW, and
those from P. densiflorawere categorized into CW, OW, and LW, as shown in Fig. 1.
Wood charcoal production
Air-dried wood samples were cut into small blocks with dimension of
2 9 2 9 4 cm3. The prepared small blocks were carbonized in an electric furnace
Fig. 1 Wood materials a TW, OW, and LW obtained from branch wood of Paulownia tomentosa; b CW,OW, and LW obtained from branch wood of Pinus densiflora
Wood Sci Technol (2016) 50:973–987 975
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(HT 16/16, Supertherm, Germany) under nitrogen atmosphere (1 kg/cm2) at 400,
600, and 800 �C. The carbonization was performed with a heating rate of 6 �C/min
from ambient temperature to the final target temperature. Once the final
carbonization temperature (400, 600, or 800 �C) was reached, the wood samples
were kept for 10 min at that temperature and then rapidly buried in cooling sand.
Methods
Measurement of the heating value, pH, char yield, chemical components,
and proximate analysis
The heating value was measured with 0.5 g of oven-dried powder using an oxygen
bomb calorimeter (Parr 6300 calorimeter) in accordance with the Korean standard
(KS E 3707 2001).
For pH measurement, oven-dried powder samples (1 g) were mixed with 100 ml
of distilled water and boiled for 10 min. After cooling to room temperature, the pH
of the supernatant solution was determined with a pH meter (InoLab, pH Level 2).
The charcoal yield was estimated by the following equation:
Yield ð%Þ ¼ W2
W1
� 100 ð1Þ
where W1 (g) is the oven-dried weight of the wood sample and W2 (g) is the oven-
dried weight of the charcoal.
For the proximate analysis, ash content, volatile matter content (VMC), and fixed
carbon content (FCC) were measured according to the KS standard (KS E ISO 562
2012; KS E ISO 1171 2012).
All the measurements were repeated five times.
The holocellulose and lignin content of TW and CW were determined by using
standard methods as follows: Wood power samples were extracted using ethanol–
benzene solution; the extractive-free powder samples were delignified using
NaClO2–acetic acid treatment (Kumar et al. 2013); holocellulose in the wood
samples was calculated using delignified powder samples according to standard
method (TAPPI 1992); and the lignin contents were measured using extractive-free
powder samples with 72 % sulfuric acid solution for 2 h (Dence 1992).
Thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC)
The combustion of wood charcoal prepared from TW and CW at 400 and 800 �C,respectively, was carried out with a TGA Q600 analyzer (TA instrument, USA)
under air atmosphere in the KNU Center for Research Facilities. This instrument
provides simultaneous measurement of weight change (TGA) and differential heat
flow (DSC). Powdered samples of ca.10 mg were continuously examined under
dynamic heating conditions from ambient temperature to 900 �C with a heating rate
of 10 �C/min.
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Scanning electron microscopy (SEM)
Samples with dimension of 10 mm (R) 9 10 mm (T) 9 5 mm (L) were prepared,
and smooth surfaces were obtained using a sliding microtome. The samples were
coated with gold using a Gressington sputter coater (ULVAC G-50DA, Japan) and
observed with a scanning electron microscope (JEOL, JSM-5510, 15–20 kV,
Japan). The changes in the cell wall dimensions were measured from SEM images
using Total Imaging Solution (IMT, I-solution Lite).
Results and discussion
Heating value
Table 1 shows the heating values of branch woods from P. tomentosa and P.
densiflora. The heating value of all samples increased as the carbonization
temperatures increased. As the temperature increased to 800 �C, the heating values
of samples increased from 18.0 to 34.0 kJ/g. It has been known that volatile matter,
such as H2O, CO, CO2, and CH4, is more released as carbonization temperatures
increased during the carbonization process, resulting in the increase in fixed carbon
content (FCC) and decrease in volatile matter content (VMC) (Han and Kim 2006;
Kumar and Chandrashekar 2013; Liu et al. 2014). Han and Kim (2006) reported that
the heating values of carbonized Korean red pine at 400 and 600 �C were 29.45 and
34.76 MJ/kg and that carbon content increased from 75.68 to 87.97 %, respectively.
Kumar and Chandrashekar (2013) observed that the heating value of samples
increased from 19.15 to 31.74 MJ/kg as the temperature increased from the ambient
temperature to 600 �C, whereas the FCC increased from 17.38 to 81.19 %, and the
VMC decreased from 80.39 to 14.79 %. Liu et al. (2014) showed that the carbon
content of charcoal exhibited a linear relationship with the heating value and the
carbon content increased from 49.59 to 74.56 % as the heating value increased from
18.52 to 27.09 kJ/g. Phan et al. (2008) also reported that the heating values of waste
Table 1 Heating values of branch woods from Paulownia tomentosa and Pinus densiflora at different
temperatures
Experimental
temperature (�C)Heating value (kJ/g)
Paulownia tomentosa Pinus densiflora
TW* OW* LW* CW* OW* LW*
Raw wood 18.5 (0.09) 18.1 (0.04) 18.1 (0.10) 19.0 (0.09) 18.5 (0.07) 18.3 (0.08)
400 28.2 (0.10) 28.3 (0.03) 27.9 (0.14) 28.9 (0.09) 29.1 (0.12) 29.3 (0.12)
600 33.2 (0.06) 33.7 (0.12) 32.8 (0.12) 33.6 (0.12) 33.1 (0.06) 33.5 (0.16)
800 34.0 (0.10) 33.8 (0.09) 33.6 (0.04) 35.2 (0.10) 33.5 (0.06) 33.5 (0.09)
TW*, tension wood; CW*, compression wood; OW*, opposite wood; LW*, lateral wood
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wood, cardboard, and textile increased as the carbonization temperature increased
from 350 to 700 �C. Moreover, Kumar et al. (2009) found that C–O bonds and C–H
bonds have lower energy than C–C bonds, and the energy value of biomass is
increased when the contents of oxygen and hydrogen are lower. Therefore, the
heating value should increase as the number of C–C bonds, which have relatively
high energy, increases.
In this study, the heating value increased significantly from ambient temperature
to 400 �C but increased only slightly from 600 to 800 �C (Table 1). The change in
heating value can be affected by increase in ash content and FCC and decrease in
the VMC as increasing the carbonization temperatures (Table 2). Moreover, heating
value of CW was found to be only slightly different to that of TW. The difference in
the biomass energy may be attributable to the major biochemical constituents,
which include cellulose, hemicelluloses, and lignin. It is known that lignin has a
heating value of approximately 27.0 MJ/kg, which is higher than that of cellulose
(17.3 MJ/kg) (Kaltschmitt et al. 2009; Yang et al. 2007). The contents of lignin
might have a considerable influence on the heating value. Demirbas (2001) also
indicated that the heating value of the biomass fuel had highly significant linear
correlation with its lignin content, where higher lignin content resulted in higher
heating value. Related to this present study, the heating value of CW charcoal was
slightly higher than that of TW charcoal. It could be related to different ash contents
and VMCs of TW and CW charcoals, since the chemical components had
decomposed after carbonization. Consequently, it is considered that heating value
could be affected by the content of lignin, ash, volatile matter, and fixed carbon.
pH
The pH values of the raw wood and wood charcoal are listed in Table 3. The pH of
all raw wood samples was weakly acidic around pH 5, but its value of the charcoals
increased as carbonization temperature increased. The wood charcoals prepared at
400 �C from both woods were acidic, whereas those at 600 and 800 �C became
basic. These results agree with those of Jo et al. (2009), who found that the pH of
charcoal from Quercus mongolica wood was weakly acidic (pH 6.74) at 300 �C and
became basic at 600 �C (pH 9.98) and 900 �C (pH 10.79). They suggested that the
Table 2 Proximate analysis of TW and CW from Paulownia tomentosa and Pinus densiflora at different
temperatures
Experimental temperature
(�C)Ash (%) VMC (%) FCC (%)
TW CW TW CW TW CW
Raw wood 0.52 (0.01) 0.19 (0.01) 75.4 (0.7) 73.2 (0.5) 16.9 (0.4) 19.9 (0.1)
400 0.67 (0.01) 0.25 (0.01) 27.4 (0.1) 26.8 (0.2) 69.9 (0.6) 70.5 (0.2)
600 0.83 (0.03) 0.43 (0.02) 21.8 (0.2) 18.8 (0.1) 76.1 (0.2) 78.7 (0.3)
800 0.95 (0.01) 0.48 (0.01) 17.0 (0.1) 14.1 (0.3) 80.8 (0.1) 84.0 (0.3)
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weak acidity of charcoal prepared at 300 �C was attributable to acidic compounds,
such as carboxylic acid and lactol. However, as the carbonization temperature
increased, the pH values of the charcoal increased because of the occurrence of
functional groups, such as pyrone. Kwon (2010) also revealed that the pH values of
carbonized woods from Quercus variabilis, Q. mongolica, and Pinus koraiensis
increased consistently from 4.6 to 9.2 as the temperature increased from 250 to
700 �C. Kai et al. (2000) reported that wood powder specimens carbonized below
500 �C were acidic, whereas those carbonized at 600 �C were basic. As the
temperature was increased, chemical components, such as cellulose, hemicelluloses,
and lignin, degraded, thereby releasing volatile matter.
The pH of CW charcoal was lower than those of OW and LW at 400, 600, and
800 �C. However, little difference between OW and LW of the two wood species
was found. The pH of carbonized CW was lower than that of TW at all
carbonization temperatures. The underlying reason for this difference could be
related to the higher lignin content of CW (Table 4). During the carbonization
process, the degradation products of lignin were mainly consisting of low molecular
syringic acid, acetic acid, and carboxylic acids (Liu et al. 2008; Pandey and Kim
2011). Namely, it was considered that pH was affected by these acidic products. The
lignin-rich CW released more acidic products than lignin-poor TW after
carbonization. Thus, CW charcoal showed lower pH value than TW charcoal.
Char yield
The char yields of the two studied woods at different carbonization temperatures are
given in Table 5. In this study, the char yields of both woods ranged from 26.79 to
31.33 % at 400 �C and decreased gradually as the carbonization temperature
increased. These results are in agreement with some previous results. Kim and
Table 4 Chemical and physical properties of TW and CW from Paulownia tomentosa and Pinus
densiflora
Samples Cellulose % (w/w) Lignin % (w/w) Hemicelluloses % (w/w) Density (g/cm3)
TW 52.95 17.85 28.10 0.36
CW 34.77 39.78 25.26 0.52
Table 3 pH of samples from Paulownia tomentosa and Pinus densiflora at different temperatures
Experimental
temperature (�C)pH
Paulownia tomentosa Pinus densiflora
TW OW LW CW OW LW
Raw wood 4.98 (0.03) 4.80 (0.01) 5.04 (0.02) 5.03 (0.05) 5.49 (0.01) 4.92 (0.02)
400 6.52 (0.04) 6.54 (0.22) 6.31 (0.11) 5.9 (0.08) 6.05 (0.01) 6.67 (0.08)
600 8.27 (0.12) 8.70 (0.05) 8.52 (0.06) 8.01 (0.15) 9.18 (0.09) 9.12 (0.04)
800 9.02 (0.08) 8.79 (0.07) 8.80 (0.09) 8.77 (0.06) 9.30 (0.02) 9.29 (0.02)
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Hanna (2006) reported similar results that the weight losses of Q. variabilis after
carbonization at 400, 600, and 800 �C were 74.4, 81.3, and 83.4 %, respectively.
Kwon et al. (2009) demonstrated that the weight loss of Q. variabilis increased as
the carbonization temperatures increased, especially at 300–350 �C. Somerville and
Jahanshahi (2015) also reported that the char yield of blackbutt charcoal decreased
from 56.7 to 28.9 % between 300 and 700 �C, especially between 300 and 400 �C.The char yield of TW was lower than that of CW at 400, 600, and 800 �C. The
CW produced the highest char yield among all studied samples at every
carbonization temperatures, while TW exhibited the lowest char yield (Table 5).
These results would be attributable to the difference in chemical composition and
density between TW and CW (Table 4). Somerville and Jahanshahi (2015) stated
that the char yield could be due to thermal decomposition of chemical components
of wood, and the higher char yield of samples could be dependent on the higher
lignin content. Moreover, Demirbas (2003) reported that there was a highly
significant correlation between the percentage of lignin content of the biomass
sample and the FCC, where the higher FCC was referred to the higher lignin content
and lower content of cellulose and hemicelluloses. The solid residue of charcoal is
mostly generated from fixed carbon; thus, the results showed the main effect of
lignin content on char yield. On the other hand, the char yield might be affected by
density of wood species. Fuwape and Akindele (1997) examined that the high char
yield of Leucaena leucocephala may be due to its high density. Cutter and
McGinnes (1981) also reported that high-density wood species were considered to
give more charcoal than low-density wood species. Thus, in this study, the char
yield of CW was higher than that of TW, since the density of CW with 0.52 g/cm3
was higher than that of TW with 0.36 g/cm3 (Tables 4, 5).
TW presented a little lower char yield than OW and LW, while CW showed a
higher char yield than OW and LW. However, no substantial difference was found
in char yield between OW and LW of both woods. As reason of this result was
considered that TW has more cellulose component than OW and LW, and CW has
more lignin component than OW and LW. Overall, lignin content, FCC, and density
are considered to play a substantial role in determining char yield.
Table 5 Char yields of samples from Paulownia tomentosa and Pinus densiflora at different
temperatures
Experimental temperature
(�C)Char yields (%)
Paulownia tomentosa Pinus densiflora
TW OW LW CW OW LW
400 27.1 (1.6) 27.9 (0.6) 26.8 (1.2) 31.3 (2.1) 30.0 (1.5) 26.6 (0.6)
600 22.7 (2.2) 24.2 (0.6) 23.2 (0.6) 28.4 (0.3) 24.3 (0.8) 24.0 (0.5)
800 21.9 (0.6) 23.2 (0.6) 22.8 (0.4) 26.6 (0.6) 23.5 (1.0) 23.5 (0.3)
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TGA and DSC analysis of wood charcoal
The TGA and DTG curves of wood charcoal from CW and TW are shown in Fig. 2,
and the ignition and peak temperature and maximum mass ratio are summarized in
Table 3. The charcoals of TW and CW prepared at 400 and 800 �C exhibited three
steps of mass loss. The first step of mass loss mainly results from the removal of
moisture and relatively high volatile compounds from the samples. The second step
of mass loss is caused by oxidative degradation, and the third one relates to the
combustion of the wood charcoal (Munir et al. 2009).
The TGA curves revealed substantial differences in the combustion behaviors
between TW and CW. The most important characteristic parameters of the entire
degradation process are the ignition temperature and the peak temperature (Haykiri-
Acma et al. 2001). The ignition temperatures of TW charcoal carbonized at 400 and
800 �C were 271 and 320 �C, while those of CW charcoal were 266 and 289 �C,respectively. The ignition temperatures of TW at both temperatures are higher than
those of CW charcoal. In the present study, there were some differences in the
amount of VMC between TW and CW charcoals (Table 2). Thus, the ignition
Fig. 2 TGA and DTG curves at different temperatures a TGA curves of TW charcoal prepared at 400and 800 �C; b TGA curves of CW charcoal prepared at 400 and 800 �C; c DTG curves of TW charcoalprepared at 400 and 800 �C; and d DTG curves of CW charcoal prepared at 400 and 800 �C
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temperature could be dependent on the differences in VMC. Kumar and
Chandrashekar (2013) reported that the variation in the ignition temperature may
be attributable to differences in VMC of wood charcoal prepared at different
temperatures. Based on the DTG curves, the maximum mass loss ratios of CW
charcoal prepared at 400 and 800 �C were 31.52 and 13.38 %/min at peak
temperatures of 392 and 416 �C, respectively. However, the values of TW charcoal
were higher than those of CW charcoal. The maximum mass loss ratios of TW
charcoal were 44.03 and 26.60 %/min at the peak temperatures of 454 and 516 �C,respectively (Table 6). The peak temperature plays a significant role in the
reactivity of wood charcoal, and the maximum mass loss ratio can be attributed to
differences in the chemical and physical properties of wood charcoal (Haykırı-Acma 2003).
The DSC curves of TW and CW charcoals at 400 and 800 �C are shown in
Fig. 3, and reaction peak temperature is listed in Table 3. Generally, DSC curves
show the decomposition and combustion of organic matter components during DSC
heating (Kloss et al. 2012). In the DSC analysis of TW charcoal, the exothermic
temperatures peaked at 472 and 529 �C, while those of CW charcoal did at 422 and
454 �C. The exothermic peaks of TW charcoal were better defined than those of
CW charcoal. According to the DSC curves, the stability of the wood charcoal
increased as the carbonization temperature increased. This increased stability could
be caused by the release of volatile matter, such as H2O, CO, CO2, and non-cyclic
hydrocarbons, from the wood charcoal (Kumar and Chandrashekar 2013).
Compared with CW charcoal, TW charcoal showed a higher peak temperature
and stability at both 400 and 800 �C. Thus, these results were mainly affected by
VMC of wood charcoal, where high VMC results in high reaction peak temperature.
Consequently, it is considered that VMC played a great role in determining the
stability of wood charcoal.
Change in cell wall morphology and cell dimensions
The SEM images of the reaction woods from P. tomentosa and P. densiflora and
their charcoal prepared at different temperatures are presented in Fig. 4. Figure 4a
shows that TW consisted of gelatinous fibers with a G-layer weakly attached to the
inner secondary wall. In contrast, CW includes intercellular spaces and helical
Table 6 Characteristic parameters determined via TGA and DSC analyses of TW and CW wood
charcoals prepared at 400 and 800 �C
Feed stock
(�C)Ignition
temperature (�C)Peak
temperature
(�C)
Maximum mass
loss ratio (%/min)
Reaction peak
temperature (�C)
TW-400 271 454 44.03 472
TW-800 320 516 26.60 529
CW-400 266 392 31.52 422
CW-800 289 416 13.30 454
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cavities (Fig. 4b, c). The original layering structures of wood cell walls were
transformed and substituted by a glazed and amorphous appearance as the
carbonization temperature increased. The cell wall layers of the middle lamella/
primary wall and secondary layers could not be identified after carbonization. This
is due to the thermal degradation of cellulose and hemicelluloses along with carbon
rearrangement into a graphite structure. These results are compatible with those of
McGinnes et al. (1971) and Kwon et al. (2009). McGinnes et al. (1971) observed
that the original microfibrillar orientation of the wood cell wall was destroyed by the
carbonization process and replaced by a smooth ‘‘amorphous-appearing’’ cell wall
structure. Kwon et al. (2009) also reported that samples carbonized between 250 and
300 �C exhibited discernible layers corresponding to the compound middle lamella
and secondary wall, while the original layering structures of the wood cell walls
transformed into a smooth and amorphous appearance as the temperature increased
from 350 �C.During carbonization, the double cell wall thickness (DCWT) of TW charcoal
changed to a greater extent than that of CW at various carbonization
temperatures (Fig. 5). The DCWT of TW charcoal at 400, 600, and 800 �Cwas contracted by approximately 47.7, 54.9, and 58.6 %, respectively, compared
to that of the raw wood samples. For CW charcoal, the DCWT decreased by
approximately 23.2, 33.6, and 34.4 %, respectively, at these temperatures. These
results are in good agreement with those of McGinnes et al. (1976). These
researchers concluded that DCWT of white oak decreased by approximately 36.5
and 50.8 % at 400 and 800 �C, respectively. In this study, TW charcoal
exhibited much larger contraction than that of CW. This phenomenon can be
explained in terms of the chemical components: TW has higher cellulose content
than CW, and CW has higher lignin content than TW. As mentioned previously,
the cellulose degradation temperature is lower than 400 �C (Yang et al. 2007).
Hence, a greater decrease in the cell wall thickness after carbonization could
result from the decomposition of cellulose at a lower temperature. Conversely,
Fig. 3 DSC curves of TW and CW charcoals obtained at different temperatures a TW charcoal preparedat 400 and 800 �C; b CW charcoal prepared at 400 and 800 �C
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Fig. 4 SEM images of raw and carbonized TW and CW at different temperatures a TW (cross section);b CW (cross section); and c CW (radial section). Scale bars 10 lm
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the lignin, which is difficult to degrade during the carbonization process, of CW
is rich. The higher lignin content may hinder the decrease in the DCWT.
Interestingly, some characteristics of the morphology of the cell walls of the
studied woods were observed after carbonization. During the carbonization
process from 400 to 800 �C, the TW G-layer disappeared. Furthermore, the
spiral striations in the tracheids of CW persisted, and the space between the
spiral striations exceeded that of the raw wood after carbonization. Additionally,
the helical cavities and open intercellular spaces could be clearly identified
(Fig. 4).
Conclusion
As the carbonization temperature increased, the heating value and pH were
increased, whereas the char yield was decreased. The differences of heating
value, pH, and char yield among reaction wood, OW, and LW were dependent
on their chemical constitution and the results of proximate analysis. The
chemical constitutions such as cellulose, lignin, and hemicelluloses were
decomposed as the carbonization temperature increased. The cell wall structure
and morphology of TW and CW have significantly changed after carbonization.
The combustion characteristics of TW and CW charcoals were different, which
may be due to the characteristics of reaction wood. Although reaction wood has
some differences in wood properties compared to normal wood, it has a great
potential for utilization as bio-charcoal. The obtained results should provide
useful information for the utilization of branch reaction wood as bio-energy
material.
Acknowledgments This study was carried out with the support from Kangwon National University in
South Korea. Yue Qi also sincerely thanks the ACES-KNU scholarship of Kangwon National University
for financial support from 2012.
Fig. 5 Changes in the cell wall thickness during carbonization at different temperatures
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